vmm vmx: Add a global bool to indicate if the host has the TSC_AUX MSR.

A future commit will remove direct access to vCPU structures from
struct vmx, so add a dedicated boolean for this rather than checking
the capabilities for vCPU 0.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37269

(cherry picked from commit 73abae4493782e44a3382b15f5563c3f400bf51f)